The present invention relates to a thin, deformable composite laminate made up of a plurality of layers comprising at least one metallic outer layer and a plastic core. The laminates are usable for manufacturing shaped parts for light-weight constructions, in particular in automobile, equipment and architectural applications.
Composites comprising a thermoplastic plastic core and at least one outer layer of aluminum are preferred materials in automobile manufacture because of the low specific weight, good rigidity, and sound and vibration damping characteristics of such materials. A major problem that exists with these materials outside architectonic applications in buildings, for example in automobile applications, is the limited formability of such composite laminate materials. These sandwich type panels exhibits a low degree of bending and low stretch-drawing limit under biaxial loading conditions during the manufacture of component parts.
Described in the German patent publication No. DE-OS 19 55 969 is a process for manufacturing metal-plastic sandwich panels by laminating a strip of thermoplastic with at least one metal strip which is subjected to a heat treatment prior to the lamination step, the lamination being performed by passing the component strips in a continuous manner between a pair of rolls. The heat treatment before lamination is performed by passing the metal strip through a furnace situated immediately prior to the laminating rolls. In the said furnace the metal strip is first heated to the temperature required to produce the desired condition in the strip. The metal strip is then, if necessary, cooled to the temperature required to join it to the plastic strip. The alloys which have been solution heat treated prior to lamination can be plastically deformed more readily, then naturally age hardened. The solution treatment temperatures lie between 350.degree. and 600.degree. C. The device for carrying out the process features a continuous heat treatment furnace situated upstream of the laminating rolls and having three successive zones for heating up, holding at temperature and cooling the metal strip.
No. EP-A1 0 019 835 discloses a plastically formable metal-plastic-metal sandwich panel and a process for its manufacture. According to this publication the total thickness of the extremely thin composites made by that process is 0.125-1.625 mm, the thickness of the metal layers being only 0.05-0.5 mm and the thickness ratio of metal to plastic less than 9:1. The bending radius of these extremely thin composites at a bend of 90.degree. C. is about the same as the total thickness of the composite.
No. EP-A1 0 019 835 contains many details of the composition of the plastic core, the use of adhesive foils, the incorporation of reinforcing elements in the plastic layer and the improved adhesion that is obtainable by a surface treatment.
Trials with outer layers of a heat treated AlMgSi alloy and a 1 mm thick plastic core of polypropylene or polyethylene revealed that the desired bending and stretch-drawing properties can not be achieved. Because of the small thickness of the metal outer layer (up to 0.25 mm), the three-layer (with two adhesive foils) or five-layer composites with thermoplastic plastic core tend to fail prematurely during shaping due to local necking of the aluminum or too little strength of the aluminum in relation to the elongation. These effects occur especially during the above mentioned bending and stretch-drawing. This shows that the thin composite laminates exhibit very poor plastic formability compared with aluminum sheet of the same rigidity.
Accordingly, it is the principal object of the present invention to develop a thin, plastically formable composite laminate having a plurality of layers including at least one metal outer layer and a plastic core wherein the laminate is suitable for manufacturing bent and/or stretch-drawn parts for light weight constructions. It should be possible to perform the deformation on conventional tools with the minimum of modification.